JP7509082B2 - Method for adjusting pH of wastewater and method for treating same - Google Patents

Description

The present invention relates to a method for adjusting the pH of wastewater and a method for treating it.

Wastewater generated at steelworks' billet rolling plants is physicochemically treated in treatment equipment that coagulates and precipitates suspended particles in the wastewater, and is then reused within the steelworks. The wastewater generated at each facility before treatment is also called raw water. The basic principle of water treatment at such treatment equipment is to first add an inorganic coagulant, polyaluminum chloride (PAC), to the raw water to form fine flocs from suspended matter (also called "SS components" or "suspended particles") that contains insoluble heavy metals in the water. Next, a polymer coagulant is added to grow the fine flocs into coarse flocs, promoting precipitation and improving the water quality.

During the PAC addition process, pH adjustment has traditionally been performed in reaction equipment such as a pH adjustment tank (neutralization tank) or reaction tank in order to convert dissolved heavy metals into insoluble matter and to keep the pH within the appropriate range for PAC reaction. Furthermore, sludge from the settling tank is collected and added at the same time as PAC in order to increase the amount of SS components that serve as the nucleus for the formation of fine flocs. Conventional methods use this method to increase the effect of fine floc formation by adding PAC (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 5-7879

In the case of the conventional water treatment method as disclosed in Patent Document 1, the pH of the raw water is adjusted by adding an alkaline pH adjuster in the reaction equipment based only on the reading of a pH meter installed in the reaction equipment, thereby accelerating the reaction of PAC and the reaction of solid-liquid separation and removal of insoluble heavy metals.
Meanwhile, in steel slab rolling plants and the like, a hot scarfer is used to perform a hot cutting process (also called "hot scarf") on steel materials in order to remove surface defects such as decarburized layers and surface flaws. In hot scarfing, if the steel contains components such as S (sulfur) and N (nitrogen), the components in the steel dissolve in the shield water, making the raw water acidic. For this reason, when a steel type with a high S content, such as free-cutting steel, is hot scarfed, the pH of the raw water drops significantly, and there is a problem that the neutralization reaction rate in the reaction equipment cannot keep up.
Therefore, the present invention has been made in consideration of the above-mentioned problems, and aims to provide a method for adjusting the pH of wastewater and a method for treating wastewater, which can stably adjust the pH of the wastewater.

According to one aspect of the present invention, there is provided a method for adjusting the pH of wastewater in a wastewater treatment facility that treats wastewater from a first drainage pit, which contains shield water used in hot scarfing of steel materials, comprising: a determination step of determining whether or not a pH pre-adjustment is necessary based on at least one of the pH of the wastewater in the first drainage pit and the type of the steel materials to be treated in the hot scarf; and an adjustment step of adding an alkaline pH adjuster to the wastewater sent from the first drainage pit to a reaction facility of the wastewater treatment facility when it is determined in the determination step that a pH pre-adjustment is necessary.

According to one aspect of the present invention, a method for adjusting the pH of wastewater and a method for treating the wastewater can be provided, which can stably adjust the pH of the wastewater.

FIG. 1 is an explanatory diagram showing a method for adjusting pH of wastewater and a method for treating wastewater according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing a method for adding a pH adjuster. 1 is a flowchart showing a method for adjusting the pH of wastewater. 1 is a graph showing the change over time in pH of the reaction tank and soluble Pb in the reservoir. 13 is a graph showing the time changes in the water reservoir, the shield water of the hot scarf, and the electrode washing water of the hot scarf dust collector. Indicates the pH of the drainage pit and the addition of pH adjuster to the neutralization reaction tank 1 is a graph showing the relationship between the sulfur content of a steel material and pH. FIG. 10 is an explanatory diagram showing a modified example of the method for adjusting pH of wastewater and the method for treating the wastewater. FIG. 10 is an explanatory diagram showing a modified example of the method for adjusting pH of wastewater and the method for treating the wastewater. FIG. 10 is an explanatory diagram showing a modified example of the method for adjusting pH of wastewater and the method for treating the wastewater. 10 is a flowchart showing a modified example of the method for adjusting the pH of wastewater. 1 is a graph showing results in an example.

In the following detailed description, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, identical or similar parts are given the same or similar reference numerals, and duplicate explanations will be omitted. The drawings are schematic and may differ from the actual product. In addition, the embodiments shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not specify the materials, structure, arrangement, etc. of the components as described below. The technical concept of the present invention may be modified in various ways within the technical scope defined by the claims.

<Method for adjusting pH of wastewater and method for treating wastewater>
A method for adjusting the pH of wastewater and a method for treating wastewater according to an embodiment of the present invention will be described with reference to Figures 1 to 3. In this embodiment, raw water, which is wastewater used and discharged from each facility in a steelworks, including a steel slab rolling plant, is treated in a wastewater treatment facility 1 shown in Figure 1.
The wastewater treatment equipment 1 includes a first drainage pit 2, a second drainage pit 3, a reaction equipment 4, and a coagulation and sedimentation equipment 5.

The first drainage pit 2 stores at least the shielding water used in hot scarfing steel in a steel billet rolling plant as wastewater. The shielding water is used in hot scarfing to prevent the burner flame from scattering, and is sprayed toward the steel downstream of the burner's spray direction. If the steel contains S or N, the S or N in the steel is oxidized by the hot scarf, and these oxides react with the shielding water to produce sulfuric acid and nitric acid. For this reason, the shielding water becomes acidic depending on the composition of the steel.

The second drainage pit 3 stores water used in the steel slab rolling plant and other facilities of the steelworks. Unlike the first drainage pit 2, the second drainage pit 3 does not store shield water used in hot scarfing of steel materials. For example, the second drainage pit 3 stores water used in finishing rolling in the steel slab rolling plant and water used in the hot scarf dust collector as drainage water. The second drainage pit 3 may also consist of two or more drainage pits instead of one.

The reaction equipment 4 is a facility that performs a pH adjustment process to adjust the pH of raw water, which is the wastewater sent from the first drainage pit 2 and the second drainage pit 3, and a reaction process to neutralize the surface charge of suspended particles in the raw water and form fine flocs. In the pH adjustment process in the reaction equipment 4, the pH of the raw water stored is measured with a pH meter 40, and an alkaline pH adjuster is added and the raw water is stirred so that the raw water has the desired pH. In this embodiment, caustic soda (NaOH) is used as an example of a pH adjuster. In addition, in the reaction process in the reaction equipment 4, an inorganic flocculant such as PAC is added to the stored raw water and stirred to form fine flocs. In this embodiment, a neutralization reaction tank 41, which is a single tank that performs both pH adjustment and reaction processes, is used as the reaction equipment 4. In the neutralization reaction tank 41, a pH adjuster is added to the raw water in the tank according to the measurement result of the pH meter 40, and an inorganic flocculant is further added. The raw water to which the pH adjuster and inorganic coagulant have been added is then stirred to promote the formation of fine flocs. The raw water that has been subjected to pH adjustment and reaction treatment in reaction equipment 4 is sent to coagulation and sedimentation equipment 5.

The coagulation and settling equipment 5 is an equipment that performs coagulation treatment of the wastewater treated in the reaction equipment 4, in which fine flocs are entangled with a polymer coagulant to form coarse flocs, and precipitation treatment of the coarse flocs in the wastewater that has been coagulated. In the coagulation treatment in the coagulation and settling equipment 5, a polymer coagulant is added to the wastewater stored in the equipment and stirred to form coarse flocs. In the precipitation treatment in the coagulation and settling equipment 5, the coarse flocs are precipitated in a tank and separated into sludge and water. The separated water is sent to a treatment water tank or the like as treated water and is reused as a water source. The separated sludge is discharged separately from the treated water. At least a portion of the water contained in the sludge may be sent again to the reaction equipment 4 and subjected to the above-mentioned series of treatments. In this embodiment, the coagulation and settling equipment 5 uses a coagulation and settling tank 51, which is a single tank that performs both coagulation and precipitation treatments. In the coagulation and settling tank 51, a polymer coagulant is added to the wastewater stored in the equipment. Then, the wastewater is stirred in the upper part of the coagulation and sedimentation tank 51 to form coarse flocs. After that, the sludge containing the coarse flocs settles in the lower part of the coagulation and sedimentation tank 51, and the sludge is separated from the treated water.

As shown in FIG. 2, the wastewater treatment facility 1 has an addition means 22 capable of adding an alkaline pH adjuster to the raw water flowing through the pipe 21, which conveys the raw water from the first drainage pit 2 to the reaction facility 4. The pH adjuster may be the same as the pH adjuster used in the reaction facility 4, and may be, for example, caustic soda. The addition means 22 may be any means capable of adding a pH adjuster, and there are no particular limitations on the structure or addition method.

In such a wastewater treatment facility 1, the raw water stored in the first drainage pit 2 and the second drainage pit 3 is sent to the reaction facility 4. At this time, a pre-adjustment process of the wastewater is performed according to the process flow shown in FIG. 3. As shown in FIG. 3, in the wastewater pH adjustment method according to this embodiment, first, it is determined whether the type of steel material to be treated by hot scarfing is a target for pH adjustment (S100, first determination step). The steel material to be pH adjusted is a steel material that may lower the pH of the shield water when hot scarfed. In other words, the determination of whether the type of steel material is a target for pH adjustment may be made depending on whether the steel material is a specific steel material that contains at least one of S and N in a predetermined amount or more, or whether the actual content of at least one of S and N in the steel material is at least a predetermined amount.

If the type of steel is determined to be subject to pH adjustment in step S100, it is determined whether the amount of steel processed in the hot scarf (e.g., the amount of steel processed per hour) is greater than or equal to a predetermined amount (S102, second judgment step).
If it is determined in step S102 that the amount of processing in the hot scarf is equal to or greater than the predetermined amount, it is determined whether the flow rate of the raw water from the first drainage pit 2 is equal to or greater than the set amount (S104, third determination step). The set amount in step S102 is a value set according to the equipment specifications, and is set appropriately according to the effect on the pH adjustment in the wastewater treatment equipment 1 of the pH of the raw water sent from the first drainage pit 2.

If it is determined in step S104 that the amount of raw water from the first drainage pit 2 is equal to or greater than the set amount, a pH adjuster is added to the raw water by the addition means 22 (S106, adjustment step). The amount (addition rate) of the pH adjuster added is set so that the pH of the raw water flowing through the pipe 21 is equal to or greater than a predetermined value (or exceeds a predetermined value). The pH of the raw water flowing through the pipe 21 is preferably set to greater than 6 if the purpose is to reduce the turbidity of the raw water, and more preferably set to 8 or greater if the purpose is to reduce dissolved lead, and even more preferably set to 9.5 or greater. The upper limit of the pH of the raw water flowing through the pipe 21 is preferably adjusted to 10 or less.

If the pH of the wastewater is 6 or less, the pH of the wastewater is too low, so the pH adjustment speed in the reaction equipment 4 may not be able to keep up. In addition, if the pH of the wastewater is 6 or less, if the steel material contains Pb (lead), the hot scarf may dissolve the Pb from the steel material contained in the wastewater into the wastewater. In water treatment, there is a standard value for Pb, so wastewater containing Pb at or above the standard value cannot be discharged from the steelworks. For this reason, from the viewpoint of dissolving Pb, it is preferable to make the pH of the wastewater greater than 6. Furthermore, in consideration of the re-dissolution of Pb and the precipitation reaction of the dissolved lead, it is preferable to make the pH of the wastewater 8 or more, and more preferably 9.5 or more. Furthermore, as described below, in consideration of the reaction efficiency in the reaction equipment 4 and the confluence of raw water from the second drainage pit 3, it is desirable to set the pH threshold to 9.5.

On the other hand, if step S100 determines that the type of steel is subject to pH adjustment, if step S102 determines that the amount of processing in the hot scarf is less than a predetermined amount, or if step S104 determines that the amount of raw water from the first drainage pit 2 is less than a set amount, it is determined that pH adjustment is not necessary, and no pH adjuster is added to the raw water by the addition means 22 (S108).

After step S106 or step S108, it is determined whether or not to end the pre-adjustment process (S110). The determination in step S110 is made based on conditions such as the operating status of the wastewater treatment equipment 1 and whether there has been an input from the operator to end the pre-adjustment process. If it is determined in step S110 that the pre-adjustment process should not be ended, the process from step S100 is repeated. On the other hand, if it is determined in step S110 that the pre-adjustment process should be ended, the pre-adjustment process ends.

In this embodiment, when determining whether or not a steel material is to be subjected to pH adjustment in step S100, in addition to the S content and N content, a determination may also be made based on the Pb content of the steel material. In this case, for example, a steel material in which at least one of the S content and N content is equal to or greater than a predetermined amount and the Pb content is equal to or greater than a predetermined amount may be determined to be a steel material to be subjected to pH adjustment.

In the wastewater treatment in the wastewater treatment facility 1, after the above-mentioned pre-adjustment treatment, the raw water from the first drainage pit 2 that has been subjected to the pre-adjustment treatment and the raw water from the second drainage pit are sent to the reaction facility 4. Then, in the reaction facility 4, a reaction process is carried out in which a pH adjustment process and a reaction process are carried out. In this embodiment, the reaction process is carried out in a neutralization reaction tank 41, which is a single device that performs the pH adjustment process and the reaction process. In the reaction process, an inorganic coagulant and an alkaline pH adjuster are added to the wastewater in the tank, and the wastewater in the tank is further stirred to generate fine flocs. In the reaction process, it is preferable to adjust the pH of the wastewater to 6 or more and 8 or less by the pH adjustment process. By setting the pH of the wastewater in such a range, the coagulation effect of inorganic coagulants such as PAC can be enhanced. In addition, since inorganic coagulants such as PAC are acidic, the pH of the wastewater decreases depending on the amount added. For this reason, it is preferable that the upper limit of the pH of the raw water adjusted in the adjustment process in the above-mentioned pre-adjustment process be set according to conditions such as the target pH of the wastewater in the reaction equipment 4, the amount of inorganic coagulant added, the amount of wastewater sent from each wastewater pit, and the pH adjustment capacity of the reaction equipment 4.

After the reaction step, the wastewater contained in the reaction equipment 4 is sent to the coagulation and sedimentation equipment 5, where a coagulation and sedimentation step is carried out. In this embodiment, the coagulation and sedimentation step is carried out in a coagulation and sedimentation tank 51, which is a single device that combines coagulation and precipitation. In the coagulation and sedimentation step, a polymer coagulant is added to the wastewater in the tank and stirred to generate coarse flocs. Furthermore, in the coagulation and sedimentation step, the coarse flocs are precipitated, and the sludge and treated water are separated.
According to the wastewater treatment method of this embodiment, the wastewater is treated through the above steps.

Here, the background to the invention according to this embodiment will be explained. In the conventional wastewater treatment method, a reaction process and a coagulation and precipitation process are carried out in a wastewater treatment facility having a reaction facility and a coagulation and precipitation facility, similar to the wastewater treatment facility 1 shown in FIG. 1. Note that the pre-adjustment process in this embodiment is not carried out in the conventional method. The inventor carried out a reaction process and a precipitation process in such a conventional wastewater treatment facility to verify the conventional wastewater treatment method. In the wastewater treatment method in this investigation, the pH of the wastewater was measured at a predetermined time interval in the reaction facility, and an alkaline pH adjuster was added to the wastewater in the reaction facility according to the pH measurement result to adjust the pH. Note that the reaction facility was composed of a single neutralization reaction tank, similar to this embodiment. In addition, the pH was adjusted so that the pH of the wastewater in the neutralization reaction tank was 7 to 7.5.

Figure 4 shows the change over time in pH in the neutralization reaction tank in the reaction facility, and the change over time in the concentration of soluble Pb in the treated water in the reservoir that stores the treated water from the wastewater treatment facility. As shown in Figure 4, although the pH never deviated from the adjustment range of 7 to 7.5, it was found that high concentrations of soluble Pb, which dissolves at a pH of 6 or less, were detected in the reservoir. In other words, the pH adjuster could not be sufficiently homogenized in the neutralization reaction tank, causing the pH value to drop in the range outside the measurement point of the pH meter. As the soluble Pb detected in the reservoir increased and then decreased between 12 and 18 hours, it was determined that the neutralization reaction in the wastewater treatment facility was not occurring sufficiently during a specific period of time.

Furthermore, to identify the period during which the neutralization reaction is deemed to have not occurred sufficiently, the frequency of soluble Pb appearing in the reservoir and changes in the conditions of the wastewater flowing into the wastewater treatment facility were investigated to see if they correlated with the soluble Pb detected in the reservoir. An investigation was conducted to see whether all rolled materials of steel types containing Pb appeared as soluble Pb in the reservoir, but it was not possible to confirm the appearance of soluble Pb in any steel types containing Pb.

Therefore, in order to confirm that the pH of the wastewater flowing into the wastewater treatment facility is related to the appearance of soluble Pb, the lead concentration of the wastewater flowing into each facility used in the experiment was investigated. The wastewater flowing into the wastewater treatment facility used in the experiment was of three types: hot scarf shield water, hot scarf dust collector electrode washing water, and finishing rolling mill scale sluice wastewater. The water quality is acidified in wastewater related to the hot scarf, which has a combustion facility, so the finishing rolling mill scale sluice wastewater was excluded from the verification candidates. Then, under conditions where lead is found in the hot scarf scalding steel, the relationship between the behavior of the lead concentration in the reservoir and the behavior of the lead concentration in the hot scarf shield water and the hot scarf dust collector electrode washing water was investigated. Figure 5 shows the time changes in the reservoir, hot scarf shield water, and hot scarf dust collector electrode washing water. As shown in Figure 5, the average lead concentration is higher in the hot scarf dust collector electrode washing water, but it was the hot scarf shield water that was confirmed to have a correlation with the behavior of the lead concentration in the reservoir. From this, it was confirmed that the hot scarf shield water had the greatest impact on water quality in the wastewater treatment equipment used in the experiment.

Figure 6 shows the pH of the drainage pit (corresponding to the first drainage pit in this embodiment) for the hot scarf shield water, and the command to add a pH adjuster to the neutralization reaction tank. The amount of pH adjuster added in a conventional reaction tank is determined by water quality laboratory tests conducted by the water treatment manufacturer prior to installation, taking into account the neutralization time and reaction time during retention. In the neutralization reaction tank used in the experiment, the amount of pH adjuster added is controlled by intermittently operating the chemical pump according to the upper and lower limits of the pH value of the neutralization reaction tank (lower limit: 7, upper limit: 7.5), and by setting a time when no pH adjuster is added, the reaction time is ensured and the pH is controlled.

Based on the above-mentioned confirmation results, the pH of the hot scarf shielding water was continuously measured, and the relationship between the pH of the neutralization reaction tank and the command to add a pH adjuster was investigated. As a result, it was confirmed that the pH of the hot scarf shielding water fluctuates greatly from 3.8 to 8.0, and when the pH dropped to around 4.0, the command to add a pH adjuster continued continuously, so sufficient reaction time could not be secured in the reaction tank. At this time, a slight decrease in electrical conductivity was also confirmed, indicating that the flocculation effect of the PAC added as an inorganic flocculant was decreasing.
Based on this, the inventor discovered that it is necessary to assist in promoting the neutralization reaction by adding a pH adjuster to wastewater containing hot scarf shield water before it flows into the reaction equipment and joins with other wastewater.

Furthermore, when adding an appropriate amount of pH adjuster to the hot scarf shielding water before measuring the pH in the reaction tank, it is necessary to always know the pH value of the hot scarf shielding water. For this purpose, it is necessary to install a pH meter in the drainage pit (first drainage pit 2 in this embodiment) for the hot scarf shielding water. However, electrode maintenance of a pH meter installed in the drainage pit for the hot scarf shielding water is difficult to maintain. For this reason, in order to consider whether there are other indicators for grasping the pH value of the hot scarf shielding water, the relationship between the hot scarf cutting conditions and pH behavior was investigated, assuming that the water quality acidification factors of the hot scarf shielding water are related to the generation of SOx and NOx by the burner.

In hot scarfing, the cutting depth is controlled by the transport time of the steel material in the scarf, and the burner combustion conditions related to the generation of SOx and NOx are not changed. For this reason, it was determined that there is no correlation with the behavior of the pH of the wastewater. Therefore, an investigation was conducted into the amount of added elements in the steel material to be cut in the hot scarf. In other words, the relationship between the pH value of the shielding water of the hot scarf and the content of various elements contained in the steel material cut by the hot scarf was investigated. As a result, it was found that the pH is low in the shielding water used when cutting steel material with a high sulfur content or nitrogen content in the steel. As an example, the relationship between the sulfur content in the steel and the pH of the shielding water is shown in Figure 7. The steel types used to collect the data in Figure 7 all have low nitrogen content, and as a result, the effect of the nitrogen content in the steel on the pH value of the shielding water is negligible. This confirmed that the sulfur content in the steel material to be cut can be used as an indicator to grasp the pH value of the shielding water of the hot scarf. Although no examples are given, it has been confirmed that the nitrogen content in steel also affects the pH value of hot scarf shield water in the same way as sulfur, that is, the pH value tends to decrease as the nitrogen content increases.

<Modification>
Although the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By referring to the description of the present invention, other embodiments of the present invention including various modifications in addition to the disclosed embodiments will be apparent to those skilled in the art. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these modifications described in this specification, either alone or in combination.

For example, in the above embodiment, the reaction equipment 4 and the coagulation sedimentation equipment 5 of the wastewater treatment equipment 1 are each composed of a neutralization reaction tank 41 and a coagulation sedimentation tank 51, which are one piece of equipment, respectively, but the present invention is not limited to such an example. For example, as shown in Figures 8 to 10, the wastewater treatment equipment 1 may have at least one of the reaction equipment 4 and the coagulation sedimentation equipment 5 composed of two pieces of equipment with separate treatment processes. As shown in Figures 8 and 10, the reaction equipment 4 may be composed of two devices, a reaction tank 42 that performs reaction processing and a neutralization tank 43 that performs pH adjustment processing. Also, as shown in Figures 9 and 10, the coagulation sedimentation equipment 5 may be composed of two devices, a coagulation tank 52 that performs coagulation processing and a sedimentation tank 53 that performs precipitation processing.

In the above embodiment, in the first judgment step, it is judged whether or not the pH adjustment is necessary based on the type of steel material to be treated by the hot scarf, but the present invention is not limited to such an example. For example, as shown in FIG. 11, a pre-adjustment process may be performed based on the measurement result of the pH of the first drainage pit 2.
11, in the preliminary treatment, first, the pH of the first drainage pit 2 is measured (S200, pH measurement step). The pH can be measured by a pH meter installed in the first drainage pit 2 or in the piping 21 from the first drainage pit 2 to the reaction facility 4.

Next, from the pH measurement result, it is determined whether or not the pH of the raw water sent from the first drainage pit 2 needs to be adjusted (S202, first determination step). The determination in step S202 may be made, for example, based on whether or not the pH of the first drainage pit 2 is equal to or lower than a threshold value (or is less than the threshold value), and it may be determined that pH adjustment is necessary when the pH of the first drainage pit 2 is equal to or lower than the threshold value (or is less than the threshold value). This threshold value is preferably 6, more preferably 8, and even more preferably 9.5. Note that the threshold value is set for the same reasons as the pH setting value in the adjustment step of the above embodiment.

When the threshold value is 6, the judgment in step S202 is that a pH adjustment is necessary if the pH of the first drainage pit 2 is 6 or less. Also, when the threshold value is 8, the judgment in step S202 is that a pH adjustment is necessary if the pH of the first drainage pit is less than 8. Furthermore, when the threshold value is 9.5, the judgment in step S202 is that a pH adjustment is necessary if the pH of the first drainage pit is less than 9.5.

If it is determined in step S202 that pH adjustment is necessary, it is determined whether the flow rate of raw water from the first drainage pit 2 is equal to or greater than a set value (S204, third determination step). The determination in step S204 may be the same as the determination in step S104.
When it is determined in step S204 that the amount of raw water from the first drainage pit 2 is equal to or greater than the set value, a pH adjuster is added to the raw water by the addition means 22 (S206, adjustment step). Note that step S206 is the same as step S106.
On the other hand, if it is determined in step S202 that pH adjustment is not necessary, or if it is determined in step S204 that the amount of raw water from the first drainage pit 2 is less than the set value, the pH adjuster is not added to the raw water by the addition means 22 (S208). Note that step S206 is the same as step S106.

After step S206 or step S208, it is determined whether or not to end the pre-adjustment process (S210). The determination in step S210 is the same as that in step S110. If it is determined in step S210 that the pre-adjustment process should not be ended, the process from step S200 is repeated. On the other hand, if it is determined in step S210 that the pre-adjustment process should be ended, the pre-adjustment process is ended.
When measuring the pH of the wastewater in the first drainage pit 2 as in this modified example, it is necessary to install a pH meter in the first drainage pit 2 or in the piping 21. However, a pH meter installed in such a location is difficult to maintain, so it is preferable to determine whether or not to pre-adjust the pH depending on the type of steel material, as in the above embodiment.

In the modified example shown in FIG. 11, when determining whether the steel material is to be pH adjusted in step S200, in addition to the S content and N content, a determination may also be made based on the Pb content of the steel material. In this case, for example, a steel material in which at least one of the S content and N content is equal to or greater than a predetermined amount and the Pb content is equal to or greater than a predetermined amount may be determined to be pH adjusted.

In addition, in the above embodiment, the addition of a pH adjuster to pre-adjust the pH is performed in the piping from the first drainage pit 2 to the reaction equipment 4, but the present invention is not limited to this example. A device for adding a pH adjuster may be provided midway in the piping from the first drainage pit 2 to the reaction equipment 4, or on the outlet side of the first drainage pit 2. By adding a pH adjuster in the piping, it can be applied to existing equipment without the need for significant equipment modifications, and installation costs can be reduced.

Furthermore, in the above embodiment, the wastewater is discharged from a steel slab rolling factory, but the present invention is not limited to this example. The facility from which the wastewater is discharged may be any other facility as long as it includes equipment for hot scarfing steel materials containing S and N.
Furthermore, in the above embodiment and modified example, in the second judgment step of step S102 and the third judgment step of steps S104 and S204, it is determined whether or not to perform pre-adjustment depending on the hot scarf processing amount and the raw water flow rate, but the present invention is not limited to such an example. If the hot scarf processing amount of the steel material to be adjusted and the raw water flow rate from the first drainage pit 2 are predetermined and do not fluctuate significantly, these judgments may be omitted.

In the above embodiment and modified examples, the wastewater treatment including the pre-adjustment process may be performed automatically. For example, the control unit (not shown) that controls the operation of the addition means 22 may acquire the hot scarf treatment conditions from a higher-level computer, acquire the pH measurement results of the first drainage pit 2, and acquire the flow rate of the piping 21 during the pre-adjustment process, and perform the pre-treatment based on the acquired results. This control unit may be, for example, a computer or a control panel.

Effects of the embodiment
(1) A method for adjusting the pH of wastewater according to one embodiment of the present invention is a method for adjusting the pH of wastewater in a wastewater treatment facility 1 that treats wastewater from a first drainage pit 2 containing shield water used in hot scarfing of steel, and includes a judgment process (S102, S200) for judging whether or not pre-adjustment of the pH is necessary based on at least one of the pH of the wastewater in the first drainage pit 2 and the type of steel to be treated by the hot scarf, and an adjustment process (S106, S206) for adding an alkaline pH adjuster to the wastewater sent from the first drainage pit to a reaction facility of the wastewater treatment facility if it is judged in the judgment process that pre-adjustment of the pH is necessary.

According to the above configuration (1), by adjusting the pH in advance, even if the pH of the wastewater in the first drainage pit 2 drops significantly due to the components contained in the steel material, the neutralization reaction rate in the reaction equipment 4 can keep up, and the pH of the wastewater can be stably adjusted. In addition, since there is no need to change the reaction equipment 4 for existing equipment, the present invention can be applied without major equipment modifications.

(2) In the configuration of (1) above, in the judgment process, when making a judgment based on the type of steel material to be processed by the hot scarf, a judgment is made based on whether the steel material is a specific steel type containing at least one of S and N.
(3) In the configuration of (1) above, in the judgment process, when making a judgment based on the type of steel material to be treated by the hot scarf, the judgment is made based on at least one of the S content and N content of the steel material.
According to the above configurations (2) and (3), there is no need to measure the pH in the first drainage pit 2, which reduces equipment costs.

(4) In the configuration of (2) or (3) above, in the judgment process, when making a judgment based on the type of steel material to be treated by the hot scarf, a judgment is made based on whether the steel material is a specific steel type containing Pb, or further based on the Pb content of the steel material.
According to the above configuration (4), the Pb concentration in the wastewater can be reduced.

(5) In any one of the configurations (2) to (4) above, in the judgment process, when making a judgment based on the type of steel material to be treated by the hot scarf, a judgment is further made based on the processing amount of the steel material to be treated by the hot scarf, in a method for adjusting the pH of wastewater described in any one of claims 4 to 6.
According to the above feature (5), the pH can be adjusted with high precision.

(6) In the configuration of (1) above, in the judgment process, when making a judgment based on the pH of the wastewater in the first drainage pit 2, if the pH of the wastewater in the first drainage pit 2 is 6 or lower, it is determined that pre-adjustment of the pH is necessary.
According to the above configuration (6), the pH of the wastewater can be adjusted more stably. In addition, when the steel material to be treated by the hot scarf contains Pb, the dissolution of Pb in the wastewater can be suppressed, and therefore the Pb content in the wastewater can be reduced.

(7) In the configuration of (1) above, in the judgment process, when making a judgment based on the pH of the wastewater in the first drainage pit 2, if the pH of the wastewater in the first drainage pit 2 is less than 8, it is determined that pre-adjustment of the pH is necessary.
According to the above configuration (7), if the steel material treated by the hot scarf contains Pb, the precipitation reaction of Pb dissolved in the wastewater can be promoted, and the Pb content in the wastewater can be reduced.

(8) In any one of the configurations (1) to (7) above, in the adjustment process, a pH adjuster is added so that the pH of the wastewater sent from the first drainage pit to the reaction equipment is greater than 6.
According to the above configuration (8), the pH of the wastewater can be adjusted more stably. In addition, when the steel material to be treated by the hot scarf contains Pb, the dissolution of Pb in the wastewater can be suppressed, and therefore the Pb content in the wastewater can be reduced.

(9) In any one of the configurations (4) to (7) above, in the adjustment process, a pH adjuster is added so that the pH of the wastewater sent from the first drainage pit 2 to the reaction equipment is 8 or higher.
According to the above configuration (9), when the steel material treated by the hot scarf contains Pb, the precipitation reaction of Pb dissolved in the wastewater can be promoted, and the Pb content in the wastewater can be reduced.

(10) In any one of the configurations (1) to (7) above, in the adjustment process, a pH adjuster is added so that the pH of the wastewater in the reaction equipment 4, including the wastewater from the first drainage pit 2, is 8 or higher.
According to the above configuration (10), when the steel material to be treated by the hot scarf contains Pb, the precipitation reaction of Pb dissolved in the wastewater can be promoted, and the Pb content in the wastewater can be reduced. In addition, the flocculation effect of the inorganic flocculant such as PAC in the reaction equipment 4 can be improved. Furthermore, if an inorganic flocculant such as PAC or PAS is used as an acidic pH adjuster, the flocculation effect of the inorganic flocculant in the reaction equipment 4 can be improved.

(11) A method for treating wastewater according to one aspect of the present invention is a method for treating wastewater in a wastewater treatment facility that treats wastewater from a first drainage pit containing shield water used when hot scarfing steel materials, and includes: adjusting the pH of the wastewater using the method for adjusting the pH of the wastewater described in any one of the configurations (1) to (10) above; after the adjustment step, in a reaction facility, adding an inorganic coagulant that neutralizes the surface charge of suspended particles in the wastewater and an alkaline pH adjuster to the wastewater and stirring the wastewater; and after the reaction step, in a coagulation and sedimentation facility of the wastewater treatment facility, adding a coagulant to the wastewater, stirring it, and precipitating the coagulated suspended particles.
According to the above configuration (11), the same effects as those of the above configurations (1) to (10) can be obtained.

A similar investigation to the investigation results of the time transition of the concentration of soluble Pb shown in FIG. 4 was conducted for the case where the present invention was applied. That is, the time transition of the pH of the neutralization reaction tank in the reaction equipment and the time transition of the concentration of soluble Pb in the treated water in the reservoir that stores the treated water treated in the wastewater treatment equipment were investigated. The results of the embodiment are shown in FIG. 12. Here, the steel type to be melt-cut by the hot scarf was the same as that shown in FIG. 4. In addition, if the sulfur content of the steel material to be treated by the hot scarf is equal to or greater than a predetermined threshold, it is determined that the material is a material to be subjected to pH adjustment (step S100 in FIG. 3), and the amount of processing by the hot scarf is equal to or greater than a predetermined amount (step S102 in FIG. 3), and further, if it is determined that the amount of raw water from the first drainage pit 2 is equal to or greater than a set amount (step 104 in FIG. 3), a pH adjuster was added to the raw water by the addition means 22. The amount (addition rate) of the pH adjuster was adjusted so that the pH of the raw water flowing through the pipe 21 was 8 to 10.

As shown in Figure 12, similar to the case of Figure 4, the pH did not deviate from the adjustment range of 7 to 7.5. The concentration of soluble Pb in the reservoir, which dissolves at a pH of 6 or less, was a maximum of approximately 120 μg/L, which is a significant reduction compared to the maximum of approximately 350 μg/L in the case of the conventional pH adjustment method shown in Figure 4.

Reference Signs List 1 Wastewater treatment facility 2 First drainage pit 21 Piping 22 Addition means 3 Second drainage pit 4 Reaction facility 40 pH meter 41 Neutralization reaction tank 42 Reaction tank 43 Neutralization tank 5 Coagulation sedimentation facility 51 Coagulation sedimentation tank 52 Coagulation tank 53 Sedimentation tank

Claims (7)

A method for adjusting the pH of wastewater in a wastewater treatment facility that treats wastewater from a first drainage pit containing shield water used in hot scarfing of steel materials, comprising:
a step of determining whether or not a pH pre-adjustment is necessary based on at least one of the pH of the wastewater in the first drainage pit and the type of the steel material to be treated by the hot scarf, the step of determining that the pH pre-adjustment is necessary when the pH of the wastewater in the first drainage pit is 6 or less, or when the steel material to be treated by the hot scarf is a specific steel type containing at least one of S and N in a predetermined amount or more;
an adjusting step of adding an alkaline pH adjuster to the wastewater sent from the first wastewater pit to a reaction facility of the wastewater treatment facility when it is determined in the determining step that the pH pre-adjustment is necessary;
A method for adjusting the pH of wastewater comprising the steps of: The method for adjusting the pH of wastewater according to claim 1, wherein in the judgment step, if the steel type of the steel material to be treated by the hot scarf contains at least one of S and N in a predetermined amount or more, and the steel type of the steel material to be treated by the hot scarf contains Pb in a predetermined amount or more, it is judged that the pH needs to be pre-adjusted. The method for adjusting the pH of wastewater according to claim 1 or 2, wherein in the judgment step, when making a judgment based on the type of steel to be treated by the hot scarf, in addition to making a judgment based on the type of steel, if the amount of the steel material to be treated by the hot scarf is equal to or greater than a predetermined amount, it is judged that a pre-adjustment of the pH is necessary. The method for adjusting the pH of wastewater according to any one of claims 1 to 3, wherein in the adjustment step, the pH adjuster is added so that the pH of the wastewater sent from the first wastewater pit to the reaction equipment is greater than 6. The method for adjusting a pH of wastewater according to any one of claims 1 to 3 , wherein in the adjustment step, the pH adjuster is added so that the pH of the wastewater sent from the first drainage pit to the reaction facility is 8 or higher. The method for adjusting a pH of wastewater according to any one of claims 1 to 3 , wherein in the adjustment step, the pH adjuster is added so that the pH of the wastewater in the reaction facility, including the wastewater from the first drainage pit, is 8 or higher. A method for treating wastewater in a wastewater treatment facility that treats wastewater from a first drainage pit containing shield water used in hot scarfing steel materials, comprising:
After adjusting the pH of the wastewater using the method for adjusting the pH of the wastewater according to any one of claims 1 to 6 ,
After the adjustment step, a reaction step of adding an inorganic flocculant and an alkaline pH adjuster to the wastewater in the reaction facility to neutralize the surface charge of the suspended particles in the wastewater and stirring the wastewater;
The wastewater treatment method includes a coagulation and sedimentation step of adding a coagulant to the wastewater and stirring the wastewater in a coagulation and sedimentation facility of the wastewater treatment facility after the reaction step, thereby precipitating the coagulated suspended particles.

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